Both are great card, but the X1900XT is better. If you want to spend extra up-front, then go for the X1900XT. If you don't want/can't spend the extra now, go for the X1800XT.
Both are good cards, but if you're willing to spend that kind of money, have you considered waiting for DX10 cards? If you don't care to wait, I would go with the x1900.
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